This invention relates to DC-to-DC power converters, and more particularly to a technique for eliminating current spikes generated by turn-off of commutating diodes in buck power converters, and in the case of a buck power converter with an isolating transformer between rectifying diodes and a switching transistor, for eliminating spikes due to turn-off of the rectifying diodes and to transformer saturation while both switching transistors are turned off.
There has been premature failure of switching power transistors in buck converters due to current spikes generated in the converter that have been determined to be a result of diode turn-off time, and in the case of a center-tapped (primary and secondary) transformer also being used, particularly in a push-pull power switching arrangement, a result of turn-off of rectifying diodes and transformer saturation while both switching transistors are turned off. The later results because the rectifying diodes have unequal currents. Large current spikes cause undue stress on the switching transistors, and that can only lead to premature failure. While there may be other causes of current spikes in a particular converter circuit, this invention is concerned with only these sources.